1. Field of the Invention
The present invention relates generally to an electrolyzer, and is directed more paticularly to an electrolyzer for use in electrodepositing a metal or alloy by fusion electrolysis by which the deposited metal, such as titanium, or an alloy can be given any desired shape such as a smooth, flat plate, a block or the like.
2. Description of the Prior Art
In the prior art electrodepositing by, fusion electrolysis, the deposited material is in a fused state, or in the form of dendrites, dendritic crystals, fine powders or sponge.
In order to avoid the foregoing problem, an improved electrodepositing method has been developed to provide an electrodeposited material which is, for example, of a smooth and flat shape. By way of example, the Japanese Pat. Nos. 212,080; 229,381; 294,943 and 726,754, some of whose inventors are the same as those of the present invention, disclose such an improved electrodepositing method.
The electrodepositing method described in the above Japanese Pat. No. 726,754 employs a fused-salt electrolyte containing at least (1) a mixture of the chloride salts of barium, magnesium, sodium and calcium having a freezing point of less than 600.degree. C. and (2) compounds of the desired metal. A portion of the electrolyte is heated to a temperature more than at least 500.degree. C. and then adjusted in its state. The higher valent compound, for example, of titanium, in the electrolyte near an electrode on which the desired metal such as titanium is electro-deposited, is maintained at less than two-thirds of the lower valent compound of the desired metal, considered in molar ratio of analyzed value at the room temperature. Under such conditions, a electrodepositing is carried out at the temperature ranging between 400.degree. and 580.degree. C.
In such electrodepositing method, the composition of fused-salt electrolyte is important. It is also important that solid state particles, which are a part of the composition of the fused salt, be suspended in the fused-salt electrolyte. Further, the ion condition of the fused salt including the ions of the desired metal, the fused condition in the fused-salt and the condition of the constituents of the precipitated crystallites are also important.
It is important that the temperature distribution of the electrolyte in the electrolyzer provide at least two portions or zones, in one of which the cathode electrode is located and in the other of which there is maintained a relatively higher temperature.
More particularly, in the electrodepositing method being described, at the electrolytic temperature the composition of fused-salt electrolyte is an excess saturation composition. Accordingly, if all of the electrolyzer is maintained at the electrolytic temperature for a long time, excessively saturated components may be precipitated as crystallites and the crystallites may grow. Therefore, even if the electrolyte is stirred, it may become gradually impossible to keep the crystallites suspended or floating in the electrolyte. Further, the constituents of the crystallites of excessively saturated components are varied in response to the cooling thereof and, accordingly, the ion condition of the desired metal is also varied. If the ion of the desired metal is multivalent, the ion condition is greatly varied by a deproportional reaction, or by the formation of a complex salt or the like. Due to this fact, even if the molar ratio of fused salts at the location within the electrolyte where the cathode electrode is immersed can be held approximately constant at the electrolytic temperature, the state of the electrodeposited material is deteriorated in the course of a long continued electrolysis.
Accordingly, in order to desirably carry out an electrolysis for a long time, it is necessary to heat the fused-salt electrolyte to more than at least the electrolytic temperature. For example in, an electrolyzer for electrodepositing metal titanium smoothly, there should be provided a low temperature portion which is maintained at an electrolytic temperature lower than the liquid of fused-salt composition and at which a cathode electrode is located, and a higher temperature portion which is held at a temperature higher than the electrolytic temperature and which heats the electrolyte to such an extent that at least a part of the crystallites of excess fused-salt composition, which are formed at the electrolytic temperature, is fused to recover the function of the fused-salt.